1. Field of the Invention
This invention relates to a differential amplifier for use in a video head amplifier employed in a home VTR, and in particular, to a differential amplifier applicable for circuit integration.
2. Description of the Prior Art
FIG. 1 shows a circuit for a conventional circuit-integrated video head amplifier. Reproduction video signals supplied by a video head are inputted via an input 2 and transmitted to the base of an emitter-grounded transistor 3 which is incorporated into the integrated circuit. The transistor 3 is also connected via its collector to a transistor 4 in a cascade connection. Transistor 4 is in turn connected to the base of a transistor 5. In other words, the collector of the transistor 3 is connected to the emitter of the transistor 4 and the collector of the transistor 4 is connected to the base of the transistor 5. Thus, the reproduction video signals supplied are inputted to the base of transistor 5.
Transistor 5 then outputs signals via its emitter to transistors 7 and 8 of a differential amplifier 6. Since transistor 8 also receives, via its base, a DC output signal from a smoothing circuit which comprises a resistor 9 and a capacitor 10, differential amplifier 6 resultantly amplifies AC components in the reproduction video signals before transmitting them into an output terminal 12. Signals outputted from the emitter of transistor 5 are also supplied to a transistor 13 and then a feedback resistor 14, and eventually fed back to the base of emitter-grounded transistor 3.
According to the circuit of FIG. 1, a reproduction video signal from a video head can be amplified before being outputted.
This circuit, however, requires an external smoothing capacitor 10, so a dedicated IC pin is required for capacitor 10.
For a four-head video amplifier, four sets of the above circuits, each including the IC pin and capacitor 10 are necessary. Although an external capacitor to be connected to an IC pin may be included in an integrated circuit, a resistor 9 having a large resistive value must be employed in such a case, to ensure the same time-constant as that which would be obtained with a capacitor externally provided because a large-capacitance capacitor cannot be incorporated into an integrated circuit.
A large value of resistor 9, however, will cause a drop in the voltage at the base of transmitter 8 when a base current thereof is caused to flow into resistor 9. As a result, a DC base voltage level becomes different between transistors 7 and 8 both included in differential amplifier 6, causing distortion of a video signal to be amplified by differential amplifier 6.